Network Security

Fundamentals of Network Security Jaydip Sen Senior Scientist Innovation Lab, Tata Consultancy Services Kolkata, INDIA Email: [email protected]

April 16, 2005

Faculty Development Program, BITM, Shantiniketan

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Agenda      

Security attacks: an introduction Cryptography: basic concepts Network security principles in use Firewall design Intrusion detection system Viruses and malicious programs

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Security Attacks: An Introduction

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Security Attacks  

Attack  any action that compromises the security of information. Four types of attack: 1. 2. 3. 4.



Interruption Interception Modification Fabrication

Basic model: S Source

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D Destination


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Security Attacks  Interruption: – Attack on availability

S

D

S

D

 Interception: – Attack on confidentiality

I

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Security Attacks  Modification:

S

D

– Attack on integrity I

 Fabrication: – Attack on authenticity

D

S I

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Security Goals

Confidentiality

Integrity

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Availability


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Passive and Active Attacks  Passive attacks – Obtain information that is being transmitted (eavesdropping). – Two types: ▪ Release of message contents:- It may be desirable to prevent the opponent from learning the contents of the transmission. ▪ Traffic analysis:- The opponent can determine the location and identity of communicating hosts, and observe the frequency and length of messages being exchanged.

– Very difficult to detect. April 16, 2005


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Passive and Active Attacks (contd..)  Active attacks – Involve some modification of the data stream or the creation of a false stream. – Four categories: ▪ Masquerade:- One entity pretends to be a different entity. ▪ Replay:- Passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect. ▪ Modification:- Some portion of a legitimate message is altered. ▪ Denial of service:- Prevents the normal use of communication facilities. April 16, 2005


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Security Services  Confidentiality  only the sender and the intended recipient should “understand” the message contents – Sender encrypts the message before sending – Receiver decrypts the message before reading

 Authentication  sender and receiver want to confirm the identity of each other

 Integrity  the message has not been altered in transit  Non-repudiation  sender cannot deny later that he/she had sent the message  Access control  prevents misuse of resources

 Availability  permanence of service, non-erasure – Denial of Service Attacks – Virus that deletes files

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Network Access Security

Network Access Security Model

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Methods of Defense  Encryption  Software Controls – Access limitations in a database. – In operating systems, protect each user from other users etc.

 Hardware Controls – For example, smartcard.

 Policies – For example, frequent changes of passwords.

 Physical Controls April 16, 2005


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Cryptography: Basic Concepts

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Introduction  

Most important concept behind network security is encryption. Two forms of encryption are in common use: 1. Private (or Symmetric) ▪ Single key shared by sender and receiver. 2. Public-key (or Asymmetric) ▪ Separate keys for sender and receiver.

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Trusted Third Party Sender Message

Receiver Channel

Key1

Message Key2

Intruder

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Symmteric (Private) Key Cryptography 

Basic ingredients of the scheme: a) Plaintext (P) ▪

Message to be encrypted

b) Secret Key (K) ▪

Shared among the two parties

c) Ciphertext (C) ▪

Message after encryption

d) Encryption algorithm ▪

Uses P and K

e) Decryption algorithm ▪ April 16, 2005

Uses C and K Faculty Development Program, BITM, Shantiniketan

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Symmteric (Private) Key Cryptography (contd..)  Security of the scheme – Depends on the secrecy of the key. – Does not depend on the secrecy of the algorithm.

 Assumptions that we make: – Algorithms for encryption/decryption are known to the public. – Keys used are kept secret.

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Simplified Model of Symmetric Encryption Shared Key K

Plaintext P

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Encryption Algorithm

Shared Key K

Ciphertext C

Decryption Algorithm


Plaintext P

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Symmetric Key Encryption- Some Points  Key distribution problem of secret key systems: – Establish key before communication. – Need n(n-1)/2 keys with n different parties. A E

B

C April 16, 2005

D


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Classical Techniques 

Broadly falls under two categories: 1. Substitution ciphers ▪ Each letter of group of letters of the plaintext are replaced by some other letter or group of letters, to obtain the ciphertext. 2. Transposition ciphers ▪ Letters of the plaintext are permuted in some form.

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Symmetric Key Cryptography Substitution Cipher: substituting one thing for another – monoalphabetic cipher: substitute one letter for another

plaintext:

abcdefghijklmnopqrstuvwxyz

ciphertext:

mnbvcxzasdfghjklpoiuytrewq

E.g.:

Plaintext: bob. i love you. alice Ciphertext: nkn. s gktc wky. mgsbc

Q: How hard to break this simple cipher?:  brute force (how hard?)  other? April 16, 2005


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Symmetric Key Encryption Substitution Ciphers 1. Caesar Cipher – Earliest known substitution cipher. – Replace each letter of the alphabet with the letter three places after that alphabet. – Alphabets are assumed to be wrapped around ( Z is followed by A, etc.). P: C:

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HAPPY NEW YEAR KDSSB QHZ BHDU


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Symmetric Key Encryption Substitution Ciphers – We can generalize the idea by replacing each letter by the kth following letter. – If we assign a number to each letter (A=1, B=2, etc), then C = E (P) = (P + k – 1) % 26 + 1 P = D (C) = (C – k + 25) % 26 + 1

– Drawback: ▪ Brute force attack is easy ▪ Try out all the 25 possible keys

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Symmetric Key Encryption 2. Mono-alphabetic Cipher – Allow any arbitrary substitution. – There can be 26! or 4x1026 possible keys. – A typical key may be: (ZAQWSXCDERFVBGTYHNMJUIKLOP) – Drawback: ▪ ▪ ▪ ▪ April 16, 2005

We can make guesses by observing the relative frequency of letters in the text. Compare it with standard frequency distribution charts in English (say). Also look at the frequency of digrams and trigrams, for which tables are also available. Easy to break in general. Faculty Development Program, BITM, Shantiniketan

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Symmetric Key Encryption 3. Poly-alphabetic Cipher – Use different mono-alphabetic substitutions as we proceed through the plaintext message. – Vigenere cipher is the best known cipher of this class. ▪ ▪

▪ ▪

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Consists of 26 Caesar ciphers, with shifts of 0 to 25. Each cipher is denoted by a key letter, which is the ciphertext letter that substitutes for the plaintext letter ‘a’. To encrypt a message, a key is needed that is as long as the message (usually, a repeating keyword). Decryption is just the reverse.


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Symmetric Key Encryption – Drawback: ▪ Key and the plaintext share the same frequency distribution of letters. ▪ The best thing would have been to use a keyword which is as large as the plaintext, and has no statistical relationship to it.

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Transposition Cipher  Many techniques were proposed under this category.  A simple scheme: – Write out the plaintext in a rectangle, row by row, and read the message column by column, by permuting the order of the columns. – Order of the column becomes the key.

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Transposition Cipher  An example P: we have enjoyed the workshop in bangalore Key:

C:

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4 3 w e n j h e h o n g

1 h o w p a

2 a y o i l

5 v e r n o

6 e d k b r

7 e t s a e

howpa ayoil ejeog wnhhn verno edkbr etsae


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Transposition Cipher – Drawback: ▪ The ciphertext has the same letter frequency as the original plaintext. ▪ Guessing the number of columns and some probable words in the plaintext holds the key.

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Symmetric Encryption Algorithms  Data Encryption Standard (DES) – Block size is 64 bits. – Key is 56 bits.

 IDEA – Block size is 64 bits. – Key size is 128 bits.

 Advanced Encryption Standard (AES) – – – –

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Also known as Rijndael cryptosystem. Block size can be 128, 192, or 256 bits. Key size can be 128, 192, or 256 bits. Brute force decryption taking 1 sec on DES, takes 149 trillion years on AES Faculty Development Program, BITM, Shantiniketan

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Data Encryption Standard DES: Data Encryption Standard  US encryption standard [NIST 1993]  56-bit symmetric key, 64-bit plaintext input  How secure is DES? – DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months – no known “backdoor” decryption approach  making DES more secure: – use three keys sequentially (3-DES) on each datum – use cipher-block chaining April 16, 2005


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Data Encryption Standard

DES operation initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation

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Public Key Cryptography 

Uses two keys for every simplex logical communication link. a) Public key b) Private key



Every communication node will have a pair of keys.

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Encryption using Public Key System B’s public key KUB

Plaintext P


B’s private key KRB

Ciphertext C


B

A

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Plaintext P


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Authentication using Public Key System A’s private key KRA

Plaintext P


Ciphertext C

A’s public key KUA


B

A

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Plaintext P


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Applications of Public Key Systems 

Three categories: a) Encryption/decryption: ▪

The sender encrypts a message with the recipient’s public key.

b) Digital signature / authentication: ▪

The sender signs a message with its private key.

c) Key exchange: ▪

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Two sides cooperate to exhange a session key.


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Requirements of Public Key Systems  Computationally easy for a party B to generate a key pair – Public key KUB – Private key KRB

 Easy for sender to generate ciphertext: ▪

C = E (M, KUB)

 Easy for the receiver to decrypt ciphertext using private key: ▪

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M = D (C, KRB) = D (E (M, KUB), KRB)


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Requirements of Public Key Systems  Computationally infeasible to determine KRB knowing KUB.  Computationally infeasible to recover message M, knowing KUB and ciphertext C.  Either of the two keys can be used for encryption, with the other used for decryption: ▪ M = D (E (M, KUB), KRB) = D (E (M, KRB), KUB)

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The RSA Public Key Algorithm  RSA Algorithm – Developed by Ron Rivest, Adi Shamir and Len Adleman at MIT, in 1977. – A block cipher. – The most widely implemented.

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The RSA Algorithm – 1. 2. 3. 4. 5. 6. 7.

Select p,q Calculate n = p x q Calculate Select integer e Calculate d Public Key Private key

Key Generation

p and q both large primes

(n)  ( p  1)(q  1)

gcd( (n), e)  1; 1  e  (n) d  e1 mod (n) KU = {e,n} KR = {d,n}

(n) is the number of positive numbers less than n and relatively prime to n (called Euler totient). April 16, 2005


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The RSA Algorithm - Encryption

 Plaintext:

M